LXR linux/arch/x86/include/asm/stackprotector.h

   1/*
   2 * GCC stack protector support.
   3 *
   4 * Stack protector works by putting predefined pattern at the start of
   5 * the stack frame and verifying that it hasn't been overwritten when
   6 * returning from the function.  The pattern is called stack canary
   7 * and unfortunately gcc requires it to be at a fixed offset from %gs.
   8 * On x86_64, the offset is 40 bytes and on x86_32 20 bytes.  x86_64
   9 * and x86_32 use segment registers differently and thus handles this
  10 * requirement differently.
  11 *
  12 * On x86_64, %gs is shared by percpu area and stack canary.  All
  13 * percpu symbols are zero based and %gs points to the base of percpu
  14 * area.  The first occupant of the percpu area is always
  15 * irq_stack_union which contains stack_canary at offset 40.  Userland
  16 * %gs is always saved and restored on kernel entry and exit using
  17 * swapgs, so stack protector doesn't add any complexity there.
  18 *
  19 * On x86_32, it's slightly more complicated.  As in x86_64, %gs is
  20 * used for userland TLS.  Unfortunately, some processors are much
  21 * slower at loading segment registers with different value when
  22 * entering and leaving the kernel, so the kernel uses %fs for percpu
  23 * area and manages %gs lazily so that %gs is switched only when
  24 * necessary, usually during task switch.
  25 *
  26 * As gcc requires the stack canary at %gs:20, %gs can't be managed
  27 * lazily if stack protector is enabled, so the kernel saves and
  28 * restores userland %gs on kernel entry and exit.  This behavior is
  29 * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
  30 * system.h to hide the details.
  31 */
  32
  33#ifndef _ASM_STACKPROTECTOR_H
  34#define _ASM_STACKPROTECTOR_H 1
  35
  36#ifdef CONFIG_CC_STACKPROTECTOR
  37
  38#include <asm/tsc.h>
  39#include <asm/processor.h>
  40#include <asm/percpu.h>
  41#include <asm/desc.h>
  42#include <linux/random.h>
  43
  44/*
  45 * 24 byte read-only segment initializer for stack canary.  Linker
  46 * can't handle the address bit shifting.  Address will be set in
  47 * head_32 for boot CPU and setup_per_cpu_areas() for others.
  48 */
  49#define GDT_STACK_CANARY_INIT                                           \
  50        [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
  51
  52/*
  53 * Initialize the stackprotector canary value.
  54 *
  55 * NOTE: this must only be called from functions that never return,
  56 * and it must always be inlined.
  57 */
  58static __always_inline void boot_init_stack_canary(void)
  59{
  60        u64 canary;
  61        u64 tsc;
  62
  63#ifdef CONFIG_X86_64
  64        BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40);
  65#endif
  66        /*
  67         * We both use the random pool and the current TSC as a source
  68         * of randomness. The TSC only matters for very early init,
  69         * there it already has some randomness on most systems. Later
  70         * on during the bootup the random pool has true entropy too.
  71         */
  72        get_random_bytes(&canary, sizeof(canary));
  73        tsc = __native_read_tsc();
  74        canary += tsc + (tsc << 32UL);
  75
  76        current->stack_canary = canary;
  77#ifdef CONFIG_X86_64
  78        this_cpu_write(irq_stack_union.stack_canary, canary);
  79#else
  80        this_cpu_write(stack_canary.canary, canary);
  81#endif
  82}
  83
  84static inline void setup_stack_canary_segment(int cpu)
  85{
  86#ifdef CONFIG_X86_32
  87        unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
  88        struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
  89        struct desc_struct desc;
  90
  91        desc = gdt_table[GDT_ENTRY_STACK_CANARY];
  92        set_desc_base(&desc, canary);
  93        write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
  94#endif
  95}
  96
  97static inline void load_stack_canary_segment(void)
  98{
  99#ifdef CONFIG_X86_32
 100        asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
 101#endif
 102}
 103
 104#else   /* CC_STACKPROTECTOR */
 105
 106#define GDT_STACK_CANARY_INIT
 107
 108/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
 109
 110static inline void setup_stack_canary_segment(int cpu)
 111{ }
 112
 113static inline void load_stack_canary_segment(void)
 114{
 115#ifdef CONFIG_X86_32
 116        asm volatile ("mov %0, %%gs" : : "r" (0));
 117#endif
 118}
 119
 120#endif  /* CC_STACKPROTECTOR */
 121#endif  /* _ASM_STACKPROTECTOR_H */
 122